This invention relates to optical devices and particularly to optical devices which suppress internal reflections of light passing through a transparent optical mass towards its outer surface at angles of incidence to the surface greater than the critical angle.
In the art of non-reflecting coatings for optical glass, carbon black is usually employed as a light-absorbing material. However, such a coating exhibits disadvantages as the refractive index of the optical glass is increased to a higher value. The amount of light totally reflected at the interface between an optical glass and a coating is increased due to the difference between the refractive index of the glass and that of the coating, so that the light reflected at the internal interface increases.
In order to overcome this defect, it is conventional to employ a coating composition having a refractive index which has a particular correlation with respect to the refractive index of the optical glass. There are, however, limits to the selection of a coating composition having a higher refractive index. Thus, a coating composition having an nd value higher than approximately 1.62 cannot be used. By nd value is meant an optical light pass length.
In addition, light of a longer wave length is absorbed to a greater extent than is light of a shorter wave length, because the longer wave length light must pass through a greater distance in the coating layer than does the shorter wave length light. For this reason, there arises the difficulty that light reflected at the internal interface becomes colored due to light in the range of relatively shorter wave lengths, when a coating having a constant coefficient of absorption with respect to light of various wave lengths, such as a black-colored coating, is used.